It frequently happens that coal, or other material, must be continuously transported overland as, for example, from a mine to a storage area, or from a storage area to a point of use. One convenient method of accomplishing this transportation is by means of an overland conveyor system. A typical overland conveyor system will consist of a series of segments in end-to-end relation stretching from the conveyor system loading, or feed, point to the final conveyor system discharge point. Each segment has an independently driven endless belt which is usually supported by idler rollers mounted on a frame resting on a ground-supported pad or other stationary support. The belts of the segments generally travel at constant, substantially equal speeds, although the belts of the segments may have to be stopped from time to time for emergency reasons such as repair. The belt of each segment (except the first and last) receives material from the adjacent upstream conveyor system segment and deposits material on the adjacent downstream conveyor system segment, so that the material deposited on the belt at the feed point of the system passes from segment to segment until it reaches the final conveyor system discharge point. If the material is fed continuously at a constant rate to the conveyor system at the conveyor system feed point, a substantially continuous layer of material will be deposited on the belts of the conveyor system with a leading edge which moves at a rate determined by the speed of the belts until it reaches the system discharge point, at which time the entire conveyor system has a layer of material thereon of constant depth stretching from the system feed point to the system discharge point. When the feed to the conveyor system is stopped, the trailing edge of the material moves from the system feed point to the system discharge point. When the trailing edge of the material discharged, the belt will be empty.
Sometimes a conveyor system must traverse undulating terrain, and frequently, in these instances, the rises in the ground will be cut and the dips will be filled in an effort to provide a more even path for the conveyor system. It will, of course, be appreciated that the construction cost of the cuts and fills increases the cost of the conveyor system, and the more extensive the construction work to provide a more even path for the conveyor system, the greater will be the added cost to the system.
In almost any conveyor system extending across undulating terrain, there will be in many of the conveyor system segments, when viewed in profile, a curve, or curves, both convex and concave, in the path of the belt of the segment which, for convenience, may be referred to as a vertical curve, or curves.
A serious problem in any conveyor system having concave vertical curves therein is any lifting of the belt in the concave curve, since the lifting will quickly fray or damage the belt. Lifting is generally caused (when the drive motor for the belt is moving the belt) by a tension in the belt upstream of the curve and a load on the belt downstream of the curve. On a downhill segment, where gravity is causing the load to drive the belt, lifting can be caused by the gravity force acting on the load on the downstream side of the vertical concave curve and a braking force exerted by the drive motor acting on the belt on the upstream side of the vertical curve. In either instance (whether the drive motor is pulling or braking the belt) the forces acting on the belt on opposite sides of the curve, act in opposite directions and lift the belt off the idlers in the curve. The forces tending to lift a belt off the idlers in a vertical concave curve will change, depending on the conditions at any given instant, and the conveyor system must be designed to overcome the lifting tendency under the most severe conditions the belt will be subjected to. In cases where the motor must generate power to drive the belt, the most severe condition will occur during acceleration of the belt from a stop, and when the leading edge of the material load is just entering the most downstream vertical concave curve of the segment. At this time, the opposing forces acting on the belt on opposite sides of the downstream curve are high, and the belt in the curve is empty, so the tendency for the belt to lift is great. In cases where the motor acts as a brake to decelerate the belt from the pulling force of gravity and inertia due to the load of material, the most severe condition occurs when a full load is on the belt downstream from the most upstream vertical concave curve of the segment, but no load is in the curve or upstream from the curve.
It has heretofore been recognized that for any given belt, and for any given load on the belt, the tendency of the belt to lift in the curve (under the worst conditions as indicated above) could be reduced by lengthening the vertical curve (that is, by increasing the radius of the curve). If this is done, a longer span of empty belt will lie on the idlers in the curve at any instant, and the increased weight of the belt will resist the forces tending to lift the belt. An empirical equation has been formulated to express the relationship between the tension, or pulling force, on the belt, the smallest radius of the curve which can be utilized without the risk of the belt lifting off the supports in the curve, and the weight of the belt, under the worst condition to which the belt is subjected. This equation is: ##EQU1## where R = radius of curvature of the belt path in feet,
T = tension in the belt adjacent the curve (upstream when the motor is supplying the power and downstream when the motor acts as a brake) in pounds, and PA1 WB = weight in pounds per linear foot of the conveyor belt.